Recently, due to the technological development of mobile devices and increasing demand therefor, demand for rechargeable batteries (or secondary batteries) has rapidly increased, and accordingly, a secondary lithium battery having high energy density and a high operating voltage, as well as excellent charge and life span characteristics has been widely used as an energy source of various electronic products as well as various mobile devices.
In general, a secondary lithium battery has a structure in which an electrode assembly and electrolyte are hermetically sealed within a battery case, and may be classified as a cylindrical battery, a prismatic battery, a pouch-type battery, or the like, according to appearance, and may be classified as a lithium ion battery, a lithium ion polymer battery, a lithium polymer battery, or the like, according to a type of electrolyte used therein. Due to the recent trend to reduce the size of mobile devices, demand for thin prismatic batteries and pouch-type batteries has been on the rise, and in particular, interest in easily deformable, lightweight pouch-type batteries has been increased.
An electrode assembly received in a battery case may be divided into a jelly-roll type electrode assembly (or a spirally-rolled type electrode assembly), a stacked-type electrode assembly, a stacked-and-folded-type electrode assembly (or a composite-type electrode assembly) according to the shape of the electrode assembly. In general, the jelly-roll type electrode assembly refers to an electrode assembly fabricated by coating an electrode active material on metal foil used as a current collector, pressing the same, cutting it into a band form having a desired width and length, partitioning a negative electrode and a positive electrode by using a separation film, and winding the same in a spiral manner. The stacked-type electrode assembly refers to an electrode assembly fabricating by stacking a negative electrode, a separator, and a positive electrode vertically. The stacked-and-folded-type electrode assembly refers to an electrode assembly fabricated by rolling or folding a continuous single layer of separator having one or more electrodes or electrode laminates comprised of negative electrode/separator/positive electrode by an elongated sheet type separation film.
However, the related art electrode assemblies known to date are generally fabricated in a manner of stacking unit cells or individual electrodes having the same size, degrading a degree of freedom in shape, to result in a great deal of design restrictions.
Thus, in order to realize various designs, methods of manufacturing a battery having a stepped portion by stacking electrodes having different sizes or unit cells have been proposed. However, batteries having a stepped portion, that have been proposed to date, are manufactured by a method in which positive electrode plates and negative electrode plates are cut to have desired areas to allow unit cells to have different areas and stacking the cut positive electrode plates and the cut negative electrode plates. At this time, since the area in each stepped portion is controllable but the thickness of the stepped portion is limited to multiples of the thickness of each stepped portion, the design freedom in designing the thickness direction of the batteries is limited.
Also, the above-described existing techniques only propose ideas that may change the design by cutting negative electrode plates and positive electrode plates to have desired sizes to form unit cells having different sizes, and stacking the cut negative electrode plates and the cut positive electrode plates, and do not propose a concrete method that may allow a battery having battery characteristics that are actually usable to be manufactured. For example, in the case of a battery having a stepped portion, although each unit cell constituting the battery and having a different size operates without error, it is common that the battery cannot actually be used, due to problems in which it is impossible to operate these batteries according to the configuration of unit cells constituting each stepped portion when the electrodes are stacked, battery capacitance is remarkably low compared to other batteries having the same volume, or severe swelling occurs at an interface between stepped portions, thus severely shortening product lifespan. However, these existing batteries having a stepped portion are not configured in consideration of the above-described problem.
Thus, there is a need for an electrode assembly capable of exhibiting large capacity characteristics while implementing various designs according to shapes of devices to which a battery cell is applied, and the development of a battery using the same are required.